We just released a cool new plugin on really-simple-plugins.com, UM Tagging, with which you can add @mention or @tagging functionality in Ultimate Member. It is integrated with the Ultimate Member social activity plugin and the Ultimate Member notifications plugin.
Just start typing @firstname, or username, email or lastname, and a list of matching users pops up. After posting the item, the username is replaced with a link to the user’s profile, and the mentioned user gets notified of the new mention, both through email and the Ultimate Member notifications add-on.
For more information see the plugin page.